$d=\sqrt{x_{1}^{2}-2x_{1}x_{2}+x_{2}^{2}+y_{1}^{2}-2y_{1}y_{2}+y_{2}^{2}}$

$x_{1}=-\sqrt{-\left(y_{2}-y_{1}\right)^{2}+d^{2}}+x_{2}$
$x_{1}=\sqrt{-\left(y_{2}-y_{1}\right)^{2}+d^{2}}+x_{2}\text{, }d\geq |y_{2}-y_{1}|$